(NEXSTAR) — Northern Arizona recently recorded its first pneumonic plague-related death since 2007. Early last year, Oregon saw its first case of the bubonic plague since 2015.

Person-to-person spread of the plague hasn’t been seen in more than 100 years in the U.S., but cases occasionally pop up. So what causes them?

The plague cases reported in the U.S. today are not exactly the same as those seen in medieval Europe. Between 1347 and 1352 alone, it’s estimated that the plague caused the deaths of at least 25 million Europeans.

Rats aboard a ship from the Crimea and Asia that docked in Sicily are blamed for bringing the bubonic plague into Europe. Once fleas ran out of host rodents in Europe - those that lacked resistance to the plague also died - they sought out humans.

Eventually, those who were sick or at risk of carrying the plague were isolated and put into quarantine, much like guidance issued during the COVID pandemic. A vaccine in the late 19th century, improved sanitation, better health practices and modern antibiotics

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